Apparatus and method for adjusting gain of photomultiplier type scintillation detectors

ABSTRACT

Photomultiplier type scintillation detectors are normalized, or adjusted to constant gain characteristics, independent of the high voltage applied to the photomultiplier. The normalized photomultiplier circuit is particularly useful in coincidence counting in liquid scintillation spectrometry, especially where the circuit is of the summation type. The inventive system has special application to apparatus for externally standardizing the spectrometer output to compensate for sample quench effects.

United States Patent 1 1 1111 3,721,824 Bristol 5]March 20, 1973 541APPARATUS AND METHOD FOR 3,101,409 8/1963 Fite ..250 715 ADJUSTING GAINOF 3,114,835, 12/1963 Packard ..250 71.5 PHOTOMULTIPLIER TYPE 3,188,4686/1965 Packard ..250/106 sc SCI-NTILLATION DETECTORS Stanley. M.Bristol, Glen Ellyn, Ill.

Packard Instrument Company, Inc., Downers Grove, Ill.

Filed: April 10, 1967 Appl. No.: 629,462

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 5/1963 Cohen et al. ..250/71.5

mag-- Primary Examiner-Morton J. Frome Att0rney-Wolfe, Hubbard, Leydig,Voit & Osann [5 7 ABSTRACT Photomultiplier type scintillation detectorsare normalized, or adjusted to constant gain characteristics,independent of the high voltage applied to the photomultiplier. Thenormalized photomultiplier circuit is particularly useful in coincidencecounting in liquid scintillation spectrometry, especially where thecircuit is of the summation type. The inventive system has specialapplication to apparatus for externally standardizing the spectrometeroutput to compensate for sample quench effects.

22 Claims, 9 Drawing Figures [1752/1/44 I'm/V0420 [Me-r e400)PATENTFUHARZOISYS 721, 24

SHEET 3 [IF 5 APPARATUS AND METHOD FOR ADJUSTING GAIN OF PHOTOMULTIPLIERTYPE SCINTILLATION DETECTORS CROSS-REFERENCE TO RELATED APPLICATIONSRobert E. Cavanaugh Ser. No. 541,721, filed Apr. 11,1966 US. Pat. No.3,499,149.

BACKGROUND OF THE INVENTION The present invention relates in general toa method and apparatus particularly useful in detecting and measuringradioactivity emanating from a radionuclidecontaining source and, moreparticularly, to a system for normalizing such apparatus to compensatefor gain changes in the transducer. In its primary aspect, the inventionis concerned with radioactivity detecting and measuring apparatusemploying two photomultiplier detectors for solid or liquidscintillation detection, and is particularly advantageous when theapparatus employs a summation-type circuit and/or externalstandardization to compensate for quench effects in the 1 sample.

Modern apparatus for detecting and measuring radioactivity has reachedan unusually high state of development. Systems are availablecommercially which offer unusual sensitivity to low energy radiation,excellent precision and accuracy, and the convenience of fully automaticoperation. Developments in apparatus and technique have been trulyremarkable.

As is frequently the case however, the apparatus which leaves themanufacturer may lose its adjustment in use due to inherent instabilityof various electrical, mechanical, or optical components.Photomultiplier tubes, which are conventionally employed as transducersor detectors in liquid and solid scintillation spectrometers, experiencea gain in amplification characteristics with time, the gain generallyincreasing rapidly at the outset and thereafter more slowly butnonetheless persistently. As will be shown, this change inphotomultiplier tube gain may have serious effects on the validity ofradioactivity detection and measurement in the types of circuitrycommonly used for radiation counting. Its effects are particularlydisturbing in counting such frequently used low energy nuclides suchastritium (maximum beta energy of 0.018 mev.) and carbon-l4 (maximumbeta energy of 0.15 mev.), especially when the radionuclide is in lowconcentration. It is accordingly a general aim and object of the presentinventionv to provide an improved radioactivity spectrometry method andapparatus in which counting errors due to changes in photomultipliertube gain are eliminated by normalizing the tube circuit, that is,returning the tube to a predetermined or standardized gain level. Anassociated object is to provide a photomultiplier tube circuit havingprovisions for normalizing the tube gain, which normalization iseffected without altering the high voltage input to the photomultipliertube.

Changes in photomultiplier tube gain are even more troublesome inradioactivity spectrometers utilizing coincidence counting, that is,simultaneous observation of scintillations produced by radioactivitydecay events with a pair of photomultiplier tube detectors, and whichrecord a count only when both detectors indicate the simultaneousreceipt of a scintillation. For

' tiplier tubes in such coincidence counting circuits so that the outputof the tubes as they observe a decay event, or the scintillationtherefrom, can be made substantially identical.

Additionally, where the coincidence counting uses summation-typecircuitry, that is, output pulses from each photomultiplier tube areadded together, unbalanced tube outputs defeat the purposes ofsummation-type circuitry and lose the advantages thereof. In particular,summationtype circuits represent a significant improvement over earliercircuits as they use the entire light output of each scintillation, withthe result that the amplitude of each event is doubled, andcorrespondingly the signal-to-noise ratio is improved by an approximatefactor of 1.4. Thus, with summation-type circuits a higher threshold canbe used. Further, summation-type circuits improve the separation orresolution between a plurality of peaks on a spectral curve containingtwo or more beta emitting radionuclides. Again, while not so limited, afurther object and advantage of the invention is to provide a systemwhereby the full benefits of summation-type circuitry are realized andwhereby difficulties resulting from unbalanced photomultiplier tubeoutputs are obviated.

Still another particular application of the present invention is inconnection with the use of external highenergy gamma-emittingradionuclides as external standards to compensate for quench effects ina sample. Such external standards provide, in comparison with internalstandardization, such important features as more rapid determination, nosample contamination, reduced opportunity for technician error, andindependence of sample activity. However the sensitivity of externalstandardization apparatus to quench effects concurrently renders theapparatus sensitive to changes in photomultiplier tube gain, and it isaccordingly still another object of the invention to provide aradioactivity spectrometer utilizing external standardization wherebythe external standardization determination, and thus the finaldetermination of sample activity, are free of aberrations caused byirregular photomultiplier tube gain.

An overall object of the present invention is to provide aphotomultiplier tube circuit particularly adapted for use inscintillation monitoring apparatus of the type having a photomultipliertube supplied with high voltage, one or more amplifiers to amplify theoutput of the photomultiplier tube, and one or more amplitudediscriminators to select a predetermined range of outputs, and toregulate the spectrometer without necessarily changing the high voltageapplied to the photomultiplier tube, the amplifier gain, or thediscriminator settings. As a consequence of attaining this object,expensive high voltage potentiometers or attenuators at thephotomultiplier tube input or output terminals are eliminated, and aconstant voltage is applied to the photomultiplier tubes so as to avoidchanges in tube characteristics due to differing input voltages.

Other objects and advantages of the invention will become apparent asthe following description proceeds, taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a generalized diagrammatic block-and-line representation of asimplified prior radioactivity spectrometer;

FIG. 2 is a diagrammatic block-and-line representation of a conventionalcoincidence type liquid scintillation spectrometer particularly suitedfor detecting and measuring the less penetrating alpha or beta particlesemanating from radioactive isotopes, and is shown in its adaptive formas capable of detecting and measuring the radiation emanating from aplurality of different nuclides within a sample;

FIG. 3 is a diagrammatic block-and-line representation of a conventionalcoincidence type liquid scintillation spectrometer utilizing coincidencecounting circuitry to enhance the performance of the spectrometer;

FIG. 4 is a graphic representation of an exemplary spectral curve for agiven beta emitting radionuclide or isotope in a sample, together withthe apparent spectral curve produced by an external gamma-emittingradionuclide, in each case showing the shift or displacement of thecurve due to quenching of the sample;

FIG. 5 is a typical quench correlation curve used in connection with anexternal standard to compensate automatically for sample quenching;

FIG. 6 is a graphic representation of a typical pulse height spectrumcharacteristic of a beta emitting radionuclide and illustrating theshift or displacement of the curve as a result of photomultiplier tubegain increase or decrease, and further illustrating the effect of suchshift on the relative number of counts passing through the A-Bdiscriminator window;

FIG. 7 is a graph similar to that of FIG. 6 but illustrating the effecton an external standardization curve of photomultiplier tube gainincrease and decrease;

FIG. 8 is a diagrammatic block-and-line representation of a multiplechannel scintillation spectrometer embodying coincidence counting,summation-type circuitry, automatic external standardization, andnormalization according to the present invention; and

FIG. 9 is a schematic diagram, partly in block-andline form, of anexemplary photomultiplier including the normalization system of thepresent invention.

While the invention is susceptible of various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular form disclosed, but, on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the invention as expressed in theappended claims.

THE ENVIRONMENT OF THE INVENTION Before treating the present inventionin detail, it will be helpful first to consider briefly the prior artbackground or environment. In radioactivity measurements, it is the mostfrequent objective to determine the rate at which decay events in anisotope present in a radioactive source occur, this rate generally beingexpressed as counts per unit time, e.g., counts per minute.

The quantity of a particular isotope present in a radioactive source isin general proportional to the rate of decay events produced by thatisotope, such rate being termed the activity level" of the source. As ageneralization, the decay events or radiation emanations from aradioactive source are, for purposes of measurement or counting,converted into corresponding voltage or current pulses which can then becounted. The pulses may be counted for a predetermined time, or apredetermined number of pulses may be counted, the ratio of countedpulses to the elapsed time period being indicative of the activitylevel. In some instances the voltage pulses may be fed to adirect-reading or recording rate meter which indicates the activitylevel.

a. General Organization and Operation of Spectrometer The spectrometerillustrated diagrammatically at 20 in FIG. 1 includes a radioactivesource 22 disposed in operative relationship to a detector orproportional transducer 24, the latter serving to convert radiation fromdecay events within the source 22 into corresponding electrical signals,e.g., voltage pulses. The transducer may be any one of a varietyavailable in the art, such for example as a sodium iodide (thaliumactivated) crystal for converting radioactive decay events into lightflashes. A transducer comprised of such a scintillation crystal and aphotomultiplier for converting light flashes into voltage pulses issuitable for detecting radiation from gamma-emitting isotopes; but stillother transducers constituting a source of pulses may be employed fordifferent specific applications. Regardless of the particular type ofdetector and transducer employed, it should have a proportionalcharacteristic, i.e., each voltage pulse will be substantiallyproportional to the energy of the decay event which produces it.

The voltage pulses so derived by the detector or transducer 24 are,however, of relatively low amplitude. It is impractical, if notimpossible, to discriminate these pulses on the basis of differences intheir amplitudes and to count or measure their rate of occurrence.Accordingly, the pulses from the detector 24 are first passed through alinear amplifier 28 and then supplied to a pulse height analyzer 29.Because the pulses received by the analyzer have been amplified and thusoccupy a fairly wide spectrum of amplitudes, the analyzer 28 may selectonly these pulses which lie above or below certain preselectedamplitudes, and pass them to a final indicating device such as a ratemeter or a scaler, the latter being shown at 30. By adjusting theamplitude band or window" of pulse amplitudes passed by the analyzer 29,the background pulses (resulting either from spurious radiation, cosmicrays, or noise in the transducer) can be substantially reduced, so thatthe count received by the scaler is comprised principally of pulsesresulting from the activity of the source being measured. The backgroundpulses to a large extent have amplitudes that fall outside theacceptance band of the analyzer after the latter has been adjusted. Thebackground count passed by the analyzer may, of course, be measured fora given time interval with no radioactive source present, and thensubtracted from subsequent scaler counts received with radioactivesources present.

Because the pulses applied to the input of the analyzer 29 haveamplitudes substantially proportional to the energy of correspondingdecay events in the source 22, the analyzer may be adjusted successivelyto pass bands of pulse amplitudes, and the energy spectrum of the decayevents in an isotope thus plotted. Once that spectrum has been measured,or is known, the analyzer 29 may be set to pass an amplitude band orwindow in which a fairly large proportion of all pulses derived from theisotope decay events are passed and counted, yet in which the backgroundor spurious pulses passed to the scaler are relatively few. Theproportion of pulses counted from source decay events to the totalnumber of decay events in the source is termed the efficiency ofcounting. It is generally accepted that the optimum counting conditionsare obtained, i.e., statistically accurate results produced withreasonably low total counts and in relatively short counting timeperiods, when the ratio E /B is made substantially a maximum (where Erepresents the efficiency of counting pulses derived from an isotope,and B represents the background count).

b. Liquid Scintillation and Coincidence Monitoring ln spectrometryinvolving isotopes which produce radiation particles having relativelylow penetrating power, and particularly with samples or sources of lowactivity levels, the detector or transducer may preferably comprise asolution of liquid scintillator into which the radioactive substance isadded. Light flashes in this solution, resulting from decay events ofthe isotope, are transmitted to a photosensitive electrical device,preferably a photomultiplier tube. Because such photomultiplier tubesare to an undesirable degree prone to produce spurious voltage pulsesdue to dark noise current which is primarily caused by thermally inducedelectron emission, it has been common practice to employ coincidencemonitoring in order to preclude counting of these spurious voltagepulses. Such a liquid scintillation spectrometer with coincidencemonitoring, and intended primarily for work with alpha and betaradiation isotopes, is diagrammiti' cally illustrated in FIG. 2.

Referring to FIG. 2, the radioactive source is shown at 32 as a sampleof isotope-containing substance dissolved or suspended in a liquidscintillator, the latter being in a container having light-transmissivewalls. Aromatic ethers are commonly employed as solvents, althoughnumerous other solvents are known in the art. Any one of numerouscommercially obtainable scintillators or fluorescent materials is alsodissolved in the solvent for the purpose of converting the radiantenergy resulting from a decay event (for example, an alpha or beta decayevent) into light energy. Finally, the sample includes the radioactivematerial, which is or contains a radionuclide or isotope, to bemeasured. The emission energy spectra of the different radionuclides mayvary greatly, each having a characteristic known spectrum. Such spectralcharacteristics are fully described in the scientific literature.

Where the maximum beta energy of the radionuclide is relatively low,typified by tritium (maximum beta energy of 0.018 mev.), carbon-l4 (0.15mev.), and up to phosphorus-32 (1.71 mev.) or so, the light flashes inthe scintillator solution are relatively weak, although proportional inintensity to the energy of the decay events which produce them. It isfor this reason that a verygsensitive light-to-voltage transducer, suchas the relatively high gain photomultiplier tube, is employed, andcoincidence monitoring is utilized to reduce the effects of noise pulsestherein.

Referring more specifically to FIG. 2, the spectrometer 31 thereillustrated includes a pair of photomultipliers 24', 2.4" which areplaced contiguous with, or adjacent to the sample 32 and energized froma variable high voltage source, not shown for reasons of simplicity andclarity. The photomultipliers 24" and 24 serve the respective functionsof providing pulses to be analyzed and pulses to monitor or gate thefirst pulses i when both photomultipliers simultaneously respond. Theoutputs of the photomultipliers 24', 24" may, if desired, bepreamplified through preamplifiers, not shown, if the output from therespective photomultipliers is insufficient, but with present day (1967)high gain photomultipliers frequently be omitted.-

In order to reduce the number of thermal noise pulses in thephotomultipliers, the sample 32, the photomultipliers 24', 24", and theoptional preamplifiers are advantageously all located within a cooledchamber or freezer, diagrammatically illustrated at 34. The outputs ofthe photomultipliers 24', 24", or of their associated preamplifiers, arerespectively coupled to optional attenuation unit 27 and one or moreoptional attenuation units 27", 27", which in turn feed to amplifiers28, 28", 28", respectively. The amplified outputs from these amplifiersare further passed to pulse height analyzers 29', 29", 29",respectively, each of which provides an input signal for a suitablelogic circuit which, for example, may simply be an AND gate. As shown inFIG. 2, the output of pulse height analyzer 299' and that of pulseheight analyzer 29" are fed to logic circuit 35" which is coupleddirectly to scaler 30". Similarly, the output of pulse height analyzer29' is also coupled, along with that of pulse height analyzer 29", tologic circuit 35" which feeds into scaler 30". Additional channels, eachcomposed of an attenuation unit, an amplifier, a pulse height analyzer,and a logic circuit connected to pulse height analyzer 29' and thence toa scaler, may be provided to accommodate additional channels forsimultaneously counting a sample containing more than two differentradionuclides.

When a decay event (for example, a beta emission) occurs in the sample32, a light scintillation is produced that is simultaneously detected byboth photomultipliers 24', 24". Correspondingly, electrical signalpulses proportional in amplitude to the energy of the decay event (i.e.,the amount of light observed by the respective photomultipliers) aresimultaneously produced at the output of the amplifiers 28', 28" (andadditional amplifiers which may be in additional counting channels).These pulses are then analyzed in pulse height analyzers 29' and 29", atleast the latter of which being adapted so that it may pass only aselected band of pulses. The analyzer 29' however preferably isconstructed and adjusted to pass all received pulses which exceed apredetermined low amplitude, and need not be restricted as tothe upperamplitude of passed pulses. Only when the analyzers 29', 29" providecoincident, or simultaneous, input pulses to the such preamplifiers mayAND gate in logic circuit 35" does the latter produce an output pulsewhich is counted by the sealer 30". If, on the other hand, coincidentinputs are not present at the AND gate, any pulse from either pulseheight analyzers 29, 29" is blocked from, and therefore not counted by,the scaler 30". By way of example, when a thermal pulse is generated ineither one of the photomultipliers, coincident input signals will not bepresent at the AND gate of logic circuit 35" and hence no count will berecorded by the scaler 30" even if the thermal pulse is within theamplitude band or window being passed by the associated analyzer.

0. Multiple Labeled Measurements and Multiple Channel Circuit As furthershown in FIG. 2, the above-described liquid scintillation andcoincidence monitoring circuit may be adapted to detect and measure theradioactivity emanating from a plurality of radionuclides within asingle sample, the detection and measuring of the radioactivity fromeach nuclide or isotope being performed simultaneously. Theabove-described circuit of FIG. 2 may, for this purpose, include anadditional counting channel composed of attenuation unit 27', amplifier28", pulse height analyzer 29", logic circuit 35" (connected to pulseheight analyzer 29'), and sealer 30", and, if desired, even more suchchannels. Such multiple channel units are adapted for the independentmeasurement of the activity levels of two or more isotopes which aresimultaneously present in a single source or sample, or which arepresent individually in mixed sources or samples, and circuits andtechniques for such detection and measurement are described, forexample, in Packard US. Pat. No. 3,114,835, patented Dec. 17, 1963.

Very briefly, in multiple channel radioactivity detecting and measuringspectrometers, the output from photomultiplier 24" is fed simultaneouslyto a plurality of channels, each containing an attenuation unit, anamplifier, a pulse height analyzer, and associated logic and scalerapparatus. In each channel the attenuation unit and/or amplifier amplifythe pulse by an amount or factor which is different from that in theother channel or channels. Depending on whether the spectral peaks ofthe respective radionuclides are sufficiently separable to permitcounting and monitoring of pulses from one radionuclide independent ofpulses contributed by the other or others, or whether the spectral peakssufficiently overlap as to require computation to determine the activityattributed to each radionuclide, multiple channel circuits are capableof simultaneously measuring the activity levels of each of severaldifferent radionuclides provided each radionuclide has a differentcharacteristic energy spectrum.

d. Summation-Type Measurement Circuit In a circuit of the type shown inFIG. 2 only one of the photomultipliers, 24", is used for detectingscintillations corresponding to decay events occurring in the sample andscintillation medium 32; the other photomultiplier 24 serves only toexclude signals from the first photomultiplier 24" that do notcorrespond to a scintillation. A significantly more efficient coincidentcounting circuit, generally termed a summation-type circuit as shown inFIG. 3, allows both photomultipliers 24', 24" to observe the sample andscintillation medium 32 and to deliver a usable output signal to thecounting channels and sealers corresponding to each decay event. Ineffect, a summation-type circuit adds algebraically the simultaneousoutput pulses of the two photomultiplier tubes 24', 24" so that theinput to the attenuation units and amplifiers is double the amplitudethat can be obtained with a circuit of the type shown in FIG. 2,described above.

Summation circuits have several advantages. First, because all of thelight output of a sample that is received by the photomultipliers isused in generating output signals to the pulse height analysis channels,more efficient counting is accomplished. Second, since the electricalsignal corresponding to each event is effectively doubled while thenoise level is unaffected, the signal-to-noise ratio is improved by afactor of 1.4 so that the pulse height analysis window (which will beexplained below) can be wider for higher counting efficiency. Third,there is apparently a closer approach to the observation of averagelight emission from a sample since each scintillation, irrespective ofwhich portion of the sample it takes place, is observed from both sidesby the respective photomultiplier tubes. Fourth, and perhaps of primeimportance in the counting of samples containing two or moreradionuclides (doublelabeled and mixed samples), the spectral peak fromeach radionuclide is made narrower, with corresponding less overlapbetween the spectrum from each nuclide and thus less interference fromother nuclides when counting a particular nuclide.

As shown in FIG. 3, the summation circuit utilizes photomultipliers 24,24" positioned to observe the sample and scintillator 32 as in thearrangement of FIG. 2. Again, preamplifiers may be used, and theassembly of sample, photomultipliers, and preamplifiers may bepositioned in a refrigerated zone, not shown. In contrast to the systemof FIG. 2 however, the output pulses delivered by photomultipliers 24',24" are not fed to separate pulse height analysis channels, but ratherare fed both to a coincidence circuit 36 and to a summation circuit 37.The coincidence circuit 36 may typically comprise an amplifier and apulse forming network associated with each line from one of thephotomultipliers, with the pulse forming networks feeding into an ANDgate which delivers an output signal only when signals from thephotomultipliers 24', 24" are simultaneous. The summation circuit 37illustratively comprises a pair of preamplifiers and a summing amplifierto sum the signal pulses received from the photomultipliers 24, 24";thus, coincident pulses from the photomultiplier are effectively doubledin amplitude by the summation circuit 37 while noncoincident pulsescorresponding to thermal noise in one of the photomultipliers remain atthe initial level.

Further in the circuit of FIG. 3, the output signals from the summationunit 37 are fed to pulse height analysis channels, of which two suchchannels are shown in the drawing. Each such channel comprises anattenuation unit or control 27", 27", an amplifier 28", 28", a pulseheight analyzer 29", 29", a logic circuit 35", 35', and a scaler 30",30". Thus, all of the pulses from summation unit 37 are received by eachpulse height analysis channel, although the only pulses transmitted tothe corresponding sealer will be those coincident pulses of properamplitude such that they pass through the window established by thepulse signal from the summation circuit 37 by the corresponding scaler30".

As indicated in FIG. 3, two or more pulse height analysis channels maybe utilized to count simultaneously a sample containing two or moreradionuclides, the method of using the summation-type circuit of FIG. 3corresponding generally to that described earlier in connection with thenon-summation type circuit of FIG. 2, and as more fully set forth in thePackard U.S.

Pat. No. 3,114,835. It will be appreciated that both circuits can beused to detect and measure the radioactivity in a sample containing aplurality of radionuclides, although the summation-type circuit of FIG.3 has certain advantages, particularly with respect to improving thecounting efficiency and to sharpening the resolution between spectralpeaks corresponding to each of several radionuclides.

e. Typical Photomultiplier Arrangement The structure and operation oftypical photomultipliers and their associated circuitry have been fullydescribed in the literature, as for example in the Packard U.S. Pat. No.3,1 l4,835,'and accordingly their principles are well understood tothose familiar with the electronic and photoelectronic arts. However, toprovide a background for an understanding of the improvedphotomultiplier circuitry constituting a feature of the presentinvention, a brief description of photomultipliers and their operationwill be presented. Attention is therefore directed momentarily to FIG.9, which shows the improved photomultiplier circuit but which is, inother respects, sufficiently general to provide a basis for an overalldiscussion of photomultipliers.

In FIG. 9, one of the photomultipliers 24' (FIG. 2, 3, and 8) is shownin diagrammatic illustration. The photomultiplier 24' includes agenerally cylindrical, opaque envelope 39 having a light transmissiveend wall disposed in proximity to the sample and scintillator (32 inFIGS. 2 and 3). Photosensitive cathode 41 is provided in thephotomultiplier 24' in proximity to the light transmissive end wall 40.Each photomultiplier 24 also includes a plurality of dynodes D1 throughD13, inclusive, which are held at progressively higher potentialsrelative to their corresponding cathode. To accomplish this, the cathodeand dynode potentials are derived from a voltage divider circuit ornetwork interconnecting the dynodes, including resistances R1 throughR13 in series with the dynodes (except for dynode D3, as to which adescription thereof will be reserved for a subsequent discussion), andresistance R14 connecting the voltage divider network to the first highvoltage dc input 42, illustratively adjustable between 1,530 and 2,450volts dc. A series of shunting capacitors C1 through C4 and C5, is inparallel with resistances R1 through R4 and R14, respectively.Capacitance C6 is connected from the high voltage dc source 42 toground. To provide adjustment of the high voltage dc source 42, thevoltage divider network is connected into an adjustable switch arm, notshown, adapted to engage a selected terminal of the high voltage dcpower source (44 in FIG. 8).

In the particular circuit of FIG. 9 a second high voltage dc source 45,illustratively adjustable between 530 and 1,450 volts dc, is connectedinto the voltage divider network at dynode D10, and a low voltage dcsource 46, illustratively of volts dc, is connected to dynode D1. Thesecond high voltage source 45 in combination with the first high voltagesource 42, provides an extremely high gain for photomultiplier 24' sothat it is unnecessary to provide for preamplification of the signalfrom the photomultiplier tube. The low voltage dc source 42 maintains arelatively high potential between the cathode 41 and the first dynode D1independently of the high voltage outputs and has been found to minimizethe effect of noise due to random fluctuations that occur in theelectron multiplication process.

By way of example, values of resistances and capacitors employed in thecircuit of FIG. 9 are presented in a table below.

TABLE I EXEMPLARY VALUES OF PHOTOMULTIPLIER TUBE CIRCUIT COMPONENTS Inthe operation of a photomultiplier 24' as a scintillation detector, thevoltage drop across resistance R13 is applied between the photosensitivecathode 41 and the first dynode D1, while the voltage drop acrossresistance R12 is applied between dynodes D1 and D2. The voltage dropacross succeeding resistance is in turn applied between each succeedingpair of dynodes (reserving momentarily a discussion of the voltage dropfrom dynode D2 to D3 and that from D3 to D4). The arrangement is suchthat when a decay event, for example, a beta emission from aradionuclide, occurs in the sample (32 of FIGS. 2, 3, and 8) it producesa light scintillation which is simultaneously detected by eachphotosensitive cathode, e.g., 41, in its respective photomultipliertube, e.g., 24'. As the light rays impinge upon the photosensitivecathode, electrons proportional in number to the energy of the light areemitted. The emitted electrons are attracted to the first dynode D1,which is at a higher voltage with respect to cathode 41, thus producingby bombardment still more electrons which are in turn attracted to thenext higher potential dynode D2. This process continues until theelectrons reach the anode 48 and thus cause current flow through, andvoltage pulses across, load resistance R15. These voltage pulses aresent to an output transformer 49 and appear at the input to theattenuation units and/or amplifiers of FIG. 2 or FIG. 3.

The total gain produced by the photomultiplier tube 24 can be varied inseveral ways. Conventionally, the magnitude of the high voltage fromsources 42 and/or 45 can be varied, e.g., by selecting an alternativeterminal on the high voltage power supply (44 of FIG. 8), or byincorporating a potentiometer or other attenuator in the high voltageinput circuit. Alternatively, a potentiometer or attenuator may beincorporated into the output from anode 48. When the photomultipliertube 24 is used in conjunction with a preamplifier, the gain of thephotomultiplier-preamplifier pair may be changed by varying thepreamplifier gain.

In order to simplify the ensuing discussion, isotope decay events willherein be referred to by way of example as beta decay events or betaemissions. It will be understood, however, that alpha and gammaradiation may be considered in the same way, even though, particularlyin the case of gamma radiation, there may be mono-energetic spectrainvolved in some instances.

f. Spectral Distributions and Optimum Counting Conditions It is wellknown that beta-emitting isotopes produce decay events whichindividually involve energies spread over a fairly wide range orspectrum. Each isotope has its own characteristic spectrum with a knownmaximum energy. A small proportion of the decay events have relativelyhigh and low energies, while the majority have energies in the middleregion between the upper and lower limits, the distribution being skewedrather markedly toward the lower limit. For a given gain of thetransducer which forms a source of voltage or current pulsesproportionally related to the energies of the decay events, theamplitude spectrum of the pulses corresponds to the energy spectrum ofthe decay events.

Referring to FIG. 4 and to the solid curve marked Sample, Unquenched",there is graphically illustrated a curve characteristic of the pulseheight spectrum of a typical beta emitting isotope or radionuclide, thecurve representing the distribution of voltage pulse heights (either atthe output of a photomultiplier tube or subsequently in theamplification circuits). Thus, for purposes of discussion, the abscissaof the graph shown in FIG. 4 may be considered as volts as a measure ofpulse height, while the ordinate is expressed in counts per unit time,or counts per minute (c.p.m.). For an actual curve, the numerical valuesof pulse height would depend upon the gain settings of thephotomultiplier and/or subsequent amplification stages (e.g., in FIGS. 2or 3), while the counts per minute scale would depend on the activitylevel of the sample. Thus, as is conventional, the area under the curveof FIG. 4 marked Sample, Unquenched" is considered to have a unitintegral, while any other curve drawn on the same chart, unlessotherwise noted, would likewise be drawn to a scale such that its areaunder the curve would be equal to that ofthe unit curve.

In the following discussion it is assumed, except as otherwise statedfor explanatory or elaborational purposes, that the sample contains onlya single radionuclide species. For a parallel discussion involvingmultiple-labeled samples, reference can be made to the Packard U.S. Pat.No. 3,114,835.

Referring once again to FIG. 4, if there were no spurious counts causedby thermal noise or background radiation, and if there were no quencheffects (to be described presently), detecting all of the counts from asample over a predetermined time period would provide a reading ofsample activity in absolute units of counts per unit time. In practicehowever this is infeasible for a number of reasons, most important ofwhich is the presence of low level background noise and high levelbackground radiation, both of which generate spurious pulses that areindistinguishable from the actual decay-produced pulses, particularly atlow and high levels of pulse height.

The effects of high and low level noise are customarily minimized byemploying amplitude discriminators as pulse height analyzers to rejectall pulses below a predetermined minimum amplitude and all pulses abovea second, higher, predetermined amplitude. Thus, in FIG. 4, amplitudediscriminators in the pulse height analyzers (e.g., 29", 29" in FIG. 2),reject all pulses having an amplitude below A and all pulses having anamplitude over B. The region between A and B is termed the window, andit will be evident that the width of the window determines the fractionof the total spectrum that is counted by the radiation detecting andmeasuring apparatus.

A given spectrometer may G or provided with two or more windows,particularly when used for counting and measuring multiple-taggedsamples. Additionally, and for reasons which will appear below, thespectrometer may have one or more integral or infinity channels, thatis, amplification channels such as those indicated by G- to-infinity andH-to-infinity on FIG. 4, where the effective window is from apredetermined minimum amplitude Gor II, respectively, and all pulsesabove that level are transmitted. Such integral windows are especiallyvaluable in connection with the use of external standards to compensatefor quenching effects.

When working with low-activity radioactive sources or samples, and whenbackground counts are appreciable, it is desirable to operate aspectrometer at or near optimum counting conditions, which serve toexclude a large proportion of background counts and to effectivelyeliminate apparent spectral shifts caused by variations in system gain(as described in conjunction with FIG. 6 below). Such optimum conditionsexist when the pulse height analyzer is adjusted so that the efficiencyof counting is high and the background counts are low, and moreparticularly when the ratio I ElB is near a maximum. This means that thewindow A-B (FIG. 4) of the pulse height analyzer should be wide, yet notso wide that background pulses included in the window become great innumber compared to the number of pulses originating from the isotope.Moreover, the window should embrace the peak portion of the pulse heightspectrum (of a beta-emitting radionuclide) in order to make theefficiency as high as possible for a given window width.

A second part of optimum counting conditions is termed balance pointoperation, more fully described in the Packard U.S. Pat. No. 3,114,835.In effect, the center of the selected window A-B is adjusted to coincideapproximately with the peak of the spectrum 50 so that lateral shifts inthe spectral curve (as will be described in connection with FIG. 6) donot appreciably affect the fraction of the total spectrum includedwithin window A-B. Otherwise stated, where the spectral peak 50 iscentered within the window A-B, lateral displacement of the curve ineither direction has a minimum effect on the integral between the limitsA and B.

g. Quenching and Quenching Compensation As suggested by the discussionof a counting window, the counts per minute indicated by a scaler (e.g.,scaler 30", 30' of FIG. 2) will be substantially less than the activityof the sample expressed as disintegrations per minute. Theproportionality factor relating sample activity (in disintegrations perminute) to the measured counts per minute is termed the countingefficiency, and is invariably less than 100 percent. Countingefficiencies of less than 100 percent are due chiefly to three factors:limitations of equipment, the deliberate exclusion of a portion of thecount by adopting the window technique, and quench.

This last element, quench or quenching, is chiefly of two types.Chemical quenching results from the presence of ingredients in thesample and scintillator (32 of FIG. 2) which interfere with theconversion of beta particles to light scintillations; organic compoundshaving a nitro group and particularly serious offenders in this respect.Color quenching is the attenuation or diminution of the brightness of ascintillation caused by colored or color-absorbing ingredients in thesample and scintillator.

The effect of either type of quenching is identical, and results in anapparent diminution of pulse height as measured by a photomultiplier oras transmitted through a pulse height analysis channel. In terms ofspectral shift, the broken line 51 of FIG. 4 illustrates thedisplacement of a spectrum caused by quenching, and it will be observedthat the new peak 52 is displaced to the left of the former peak 50. Inkeeping with the conventional maintenance of constant area under thespectral curves, the new peak 52 is shown higher than the old peak 50,although it will be appreciated that, for a given sample, there is nochange in disintegrations per minute as a result of quenching; the onlychange is an apparent diminution of measured counts per minute.

Systems and techniques have been devised for compensating for quencheffects, and as will be explained subsequently, the inventive techniqueof normalization disclosed herein is of particular utility in connectionwith certain types of quench compensation apparatus and procedures.

1. Internal Standard Perhaps the earliest technique of quenchcompensation is by the use of an internal standard, that is, theprocedure of determining the activity of a sample in a scintillator,adding a known quantity of a known standard, re-determining theactivity, and computing the unquenched activity of the original samplefrom the ratio of the difference in the two measurements and theexpected difference based on the known activity of the standard.

By way of example, assume that a test sample is placed in thespectrometer and records an activity of 50,000 counts in 1 minute(50,000 c.p.m.). Obviously the 50,000 counts are not truly indicative ofthe number of decay events that occurred in one minute, since somecounts are not recorded due to instrument limitations, others becausethey fall below the detection threshold, still others because they areoutside the window, and finally others because of quenching.Consequently, a spectrometer will always operate at below 100 percentefficiency.

Assuming that there is no quenching, it is relatively easy to determinethe efficiency. This could, for example, be done by first inserting astandard into the counting chamber which is known to undergo, say,100,000 decay events per minute. If a standard is then counted andrecords 50,000 counts in one minute, the machine is operating at 50percent efficiency. Therefore, if it were not for quenching, the 50,000counts per minute recorded from the test sample is representative of anisotope having an activity level of 100,000 counts per minute If thesample is quenched however an additional compensation must be madebecause the quenching will reduce the counting efficiency to a levelbelow 50 percent. In the technique known as internal standardization,once the count for the unknown test sample is recorded at, say, 50,000counts per minute, the technician adds a known amount of the sameradionuclide to the sample, assuming, in this case, one having anactivity of 50,000 disintegrations per minute. The sample (with addedstandard) is then recounted. Had the sample been unquenched, and knowingthe efficiency of the machine is 50 percent, it would be anticipatedthat the addition of 50,000 disintegrations per minute into the sampleshould produce 25,000 additional counts in the second countingoperation. In other words, the second counting operation should produce75,000 counts, of which 50,000 are contributed by the radionuclideoriginally present in the sample and the other 25,000 are contributed bythe known standard. However, if there is quenching present, the secondcount will be less than 75,000. It may, for example, be 62,500. Thus,the technician will know that he has lost an additional 25 percent ofthe pulses because of quenching and he may therefore determinearithmetically the true activity level of the sample.

2. External Standard--Straight Count The technique of internalstandardization is rather cumbersome and laborious; it is slow, theoriginal sample is invariably contaminated by the standard, there isopportunity for technician error, and frequently different standardsmust be used with different samples where the samples have widelydiffering activities. To avoid these limitations, the technique ofexternal standardization has been developed, and for a completedescription of one type of external standardization reference may bemade to Packard U.S. Pat. No. 3,188,468, issued June 8,1965.

Briefly, for external standardization, a sample is first counted alone,a high energy gamma-emitting source is placed near the sample, and thetwo re-counted together. By relating the actual change in measuredcounts to the expected change produced by the external standard (asdetermined by previously counting the external standard in the presenceof an unquenched standard), the un-quenched activity of the sample canthen be computed.

Some of the above concepts will become more clear by referring again toFIG. 4. It is there seen that curve 54, marked External Standard,Unquenched, and produced by Compton interaction of the gamma rays fromthe external standard with the material in the sample and scintillator,is shifted to the position indicated diagrammatically by broken curve 55as a result of quenching. This shift is much the same as the shift ofthe Sample, Unquenched shift to the new curve Sample, Quenched" 51produced by quenching. Thus, by one simple technique, known as thestraight count method, determining the change in counts produced only bythe external standard at any predetermined window or infinity channel(e.g., the G-to-infinity channel) it is possible to compute the effectof quenching on the sample itself.

In somewhat more detail, the straight count method can be illustratedwith reference to FIG. 5. Ignoring momentarily the information on FIG. 5included within parentheses, the figure depicts a calibration curve foran external standard and a sample of known activity as a function of thedegree of quenching. To prepare a curve such as that of FIG. 5 a seriesof samples having the same known activity is made up, respectivelysamples I, II, III, IV, the latter three containing progressivelyincreased amounts of an ingredient known to produce quenching.Advantageously, these samples contain the same scintillator, solvent,and radionuclide as the unknown" samples. These samples are then countedin the presence of the external standard and a curve such as that ofFIG. 5 plotted. Then, using the same channel or channels that wereemployed in preparing FIG. 5 to count the pulses produced by theexternal standard in the presence of an unknown sample, the plot of FIG.5 immediately yields a numerical value for the counting efficiency. Whenthis efficiency is divided into the measured activity of the unknownsample counted in the absence of the external standard, the quotient isthe true activity of the sample in disintegrations per minute.

3. Net External Standard Ratio A further improvement of the technique ofexternal standardization entails the use of two channels, rather thanone, to determine the amount of quenching of radiation produced by theexternal standard. This technique, known as the net external standardratio procedure, is particularly adaptable to the automatizing ofexternal standard quench determinations.

Again inviting attention to FIG. 4, the selected channels fordetermining quenching effects of radiation from the external standardare selected at two levels, preferably each above the maximum activitylevel of any beta-emitting radionuclide in the sample. Thus, asillustrated in the figure, one channel is I-I-to-infinity while theother channel is G-to-infinity. For practical purposes, it is preferablethat the total number of counts produced by the external standard on anunquenched blank be in the ratio of 2:] from the G-toinfinity channeland the H-to-infinity channel. Otherwise stated, with an externalstandard and an unquenched blank, the G-to-infinity channel will recordtwice as many counts as will the H-to-infinity channel.

As will be apparent from FIG. 4, as quenching occurs the ExternalStandard, Unquenched" curve originally at 54 will be displaced to thenew position 55 marked External Standard, Quenched". Moreover, the ratioof counts received by each of the two infinity channels will no longerbe the same. This change of ratio provides a versatile procedure forcompensating against the effects of quenching.

Adverting attention again to FIG. 5, but this time including theinformation within parentheses, the same type of calibration curve maybe used as was employed in the straight count method. For conveniencehowever, it is preferable that the ratio of the counts through theG-to-infinity and the I-I-to-infinity channels be converted by anarbitrary factor to unity, so that a change in the ratios provides animmediate quantitative indication of the degree of. quenching. Also, itis usually more convenient to employ the ratio of H-to-infinity toG-toinfinity, rather than vice versa.

Suitable circuitry for utilizing the net external standard ratiotechnique has been described elsewhere, as for example in Robert E.Cavanaugh, Jr. Application Ser. No. 54l,72l, filed Apr. 11, 1966, andaccordingly only a brief description thereof as it applies to thepresent invention is here furnished. Turning to FIG. 8, an exemplaryexternal standardization circuit, shown schematically as circuit 56, istherein depicted in combination with a summation type coincidencecounting circuit which also employs normalization according to thepresent invention. The normalization feature is best reserved for laterdiscussion, but in all other significant respects the system of FIG. 8corresponds to the summation-type coincidence circuit system previouslydescribed in connection with FIG. 3, except for the addition of externalstandardization and of an additional pulse height analysis channel(attenuation unit 27", amplifier 28", pulse height analyzer 29", andlogic unit 35'). The various logic units 35", 35, and 35" feed into acontrol logic, scaler, computer, and readout system shown schematicallyas block 58. This system of block 58 also controls the operation of theautomatic external standard placement mechanism, or in-out control 59,adapted to position the external standard near the sample during apreselected period, e.g., 0.5 minutes, of automatic external standardcounting in conjunction with the spectrometric analysis of each sample.The apparatus for effecting automatic external standardization in-outcontrol, schematically shown as 59 in FIG. 8, is more fully exemplifiedin Packard US. Pat. No. 3,188,468.

As shown in FIG. 8, all pulses received by either or bothphotomultipliers 24' and 24" are summed in summation circuit 37 and arefed to the three pulse height analysis channels as well as to theexternal standardization system 56. The pulse height analysis channelswere described previously in connection with FIG. 3 and no furtherdescription is warranted.

External standardization system 56 is comprised of an amplifier 61, a1:2 attenuation circuit 62, and a pair of discriminators, H-to-infinitydiscriminator 64 and G- to-infinity discriminator 65, the latter twobeing in parallel relationship. These discriminators respectively passpulses of heights corresponding to the I-I-to-infinity and theG-to-infinity channels shown in FIG. 4. Additionally, theI-I-to-infinity discriminator (or pulse height analyzer) 64 is madeadjustable so that it will pass exactly half as many counts (from theexternal standard irradiating an unquenched blank) as will theG-to-infinity discriminator 65. Further, to accommodate a moreconveniently usable number of pulses, the l-I-to-infinity discriminator64 is followed by a frequency divider 66 which divides the output pulsesfrom l-I-to-infinity discriminator 64 by a factor of two, and theG-to-infinity discriminator 65 is followed by a frequency divider 68which effects a division by four. Thus, inasmuch as pulses from theG-to-infinity discriminator 65 are divided by a factor twice as great asthe pulses from I-I-to-infinity discriminator 64, both discriminatorswill ultimately deliver the same number of counts to the control logic,scaler, computer, and readout system 58 (with the external standardirradiating an unquenched standard).

h. Effect of Amplification Gain Change Although the system as describedabove is capable, both in theory and in practice, of accuratelydetecting and measuring radioactivity when all components arefunctioning properly, the qualification that all components are sofunctioning is an important condition that is not infrequently violated.While the electronic components currently available possess unusualstability, this stability is not shared by the photomultipliers. Asindicated previously, photomultipliers exhibit an increase inamplification gain that can be quite marked at the outset and, evenafter extended use, can depart from their normal, or specified, gaincharacteristics. Moreover, such departure may reduce or eliminate manyof the benefits from coincident counting, summation-type circuitry, andexternal standardization.

This may be illustrated with reference to FIGS. 6 and 7, respectivelyshowing (in solid lines) the spectral curve for a given beta-emittingisotope and the effective spectral curve from an external standard. Bothfigures show, in broken lines designated as Gain Increase, the shift ordisplacement of the respective curves as photomultiplier tube gainincreases, and in dotted lines marked Gain Decrease the curve shifts asthe gain decreases. Although the magnitude of these shifts has beenexagerated for illustrative purposes, it is readily apparent that ashift in the spectral curve caused by gain increase or decrease canradically affect the counting efficiency even with balance pointoperation by displacing the spectral curve with respect to the A-Bwindow. Also, and as shown in FIG. 7, an increase or decrease inphotomultiplier tube gain canand doesprofoundly affect the sensitiveI-I-to-infinityzG- to-infinity ratio employed with the net externalstandard ratio method using external standardization.

To state the matter another way, even the most ingenious apparatus andthe most careful techniques can be frustrated by a change in gain ofeither or both of the photomultipliers.

NORMALIZATION ACCORDING TO THE INVENTION The system of the inventionprovides an apparatus and method for restoring the photomultiplier tubeor tubes to a predetermined normal, or standardized, operatingcharacteristic. While not so limited in its application, the inventionhas particular applicability to spectrometers using coincidencecounting, summationtype circuitry, and/or external standardization,where it permits the full benefits of these procedures to be realized.

In brief over-view, normalization permits the gain of a photomultiplierto be adjusted, and therefore returned to a predetermined normalcondition, without necessarily altering the high voltage power input tothe photomultipliers. It permits the photomultiplier or photomultipliersto be maintained at a constant gain level irrespective of the tendencyof such gain to change with time. Thus, the distortion in countingefficiency described in connection with FIG. 6 and the distortion inexternal standardization as discussed in connection with FIG. 7, amongother things, may be substantially eliminated.

Optimally, normalization according to the invention is used incombination with a spectrometer having dual channel coincidencecounting, summation-type circuitry, and automatic externalstandardization. Accordingly, one such system is exemplified by thecircuit of FIG. 8, much of which has been described previously.

By way of recapitulation, the number 1 and number 2 photomultipliers,shown respectively as 24" and 24,

observe the sample and scintillator 32, which is emitting lightscintillations in response to the decay of a radionuclide in the sample.Each photomultiplier is supplied with a high voltage from high voltagesource 44, and as a result of the functioning of the dynodes (FIG. 9) ineach respective photomultiplier, the photomultipliers deliver outputpulses proportional to the intensity of light observed by eachphotomultiplier. These output pulses are simultaneously fed to acoincidence or logic circuit 36, which gates the individual logiccircuits 35", 35", 35" in the respective pulse height analysis'channelsuOutput pulses from the photomultipliers 24', 24" are also fed to thesummation circuit 37, which algebraically sums the pulses from thephotomultipliers and delivers its output pulses to (l) the several pulseheight analysis channels, each composed of an attenuation unit, anamplifier, a pulse height analyzer, and a logic circuit and (2) theexternal standardization circuit 56. Thus, in the circuit depicted inFIG. 8, each of the pulse height analysis channels and the externalstandardization circuit 56 are at all times supplied with voltage pulsesfrom the summation circuit 37. The control logic, scaler, computer andreadout system 58 determines which of the various channels will beconnected in circuit to the readout system using principles that form nopart of the present invention. Additionally, the system 58 isadvantageously provided with a computer for determining and printing outthe ratio of the external standardization circuit 56, that is, twice theratio of output pulses from H-to-infinity discriminators 64 (lessdeduction for background noise) to the output of G-toinfinitydiscriminator 65 (again less a deduction for background noise). Thisratio, as explained earlier, will be unity for an unquenched sample.

Considering normalization only from a functional standpoint anddeferring momentarily the manner in i tipliers has a higher gain thanthat intended. It will be evident, from a consideration of FIG. 7, thatthe apparent spectral curve will follow the broken "Gain Increase" curverather than the proper unbroken curve that would be expected whencounting an external standard and an unquenched standard. Further, thepulse ratio from the external standardization circuit 56 will be greaterthan unity because the I-I-to-infinity channel will be receiving morethan half as many pulses as are counted through the G-to-infinitychannel.

Additionally, it will also be apparent that the shift of the curve inFIG. 7 will be dependent on gain increases of each photomultiplier tube24', 24" for the reason that the amplifier 61 in the externalstandardization circuit 56 is fed pulses from the summation circuit 37that represent the sum of the pulses from each photomultiplier.

In the schematic circuit of FIG. 8, a normalization circuit 70 isprovided which, in a manner to be discussed below, is capable ofaltering the gain of each photomultiplier 24, 24" (or associatedcircuitry) independent of the other photomultiplier. This normalizationcircuit 70 is associated with a shunt switch 71 around the 1:2attenuation unit 52 in the external standardization circuit 56 so thatwhen one of the photomultipliers is rendered inactive, pulses from theother, active, photomultiplier will not be attenuated in the externalstandardization circuit 56 and will accordingly have the same amplitudeas would the summed pulses from both photomultipliers 24', 24" with theattenuation circuit 52in operation.

Still considering normalization only from a functional standpoint, it ispossible, with the circuit of FIG. 8, to determine whether eachphotomultiplier is functioning normally or whether it has experienced again increase or decrease. To this end, the external standard gammaemitter is inserted near an unquenched standard (containing ascintillator but having no activity). Then with one of thephotomultipliers deactivated and with the 1:2 attenuation circuit 62shorted out, the spectrum will correspond to that shown in FIG. 7: thesolid line if the active photomultiplier has a normal gain, the dashedline if it has experienced a gain increase, and the dotted line if therehas been a gain decrease. The gain increase or decrease will, moreover,be reflected as a change in the ratio of counts detected by theI-I-to-infinity discriminator and that detected by the G-to-infinitydiscriminator (after multiplying by the factor of two to maintain theexpected ratio at unity). Thus, by regulating or adjusting the output ofthe photomultiplier tube undergoing normalization to establish a ratioas close as desired to unity, the photomultiplier will then have itsgain restored to the desired gain represented by the solid line of FIG.7.

By the same token, the same procedure is repeated with the oppositephotomultiplier engaged in circuit and with the previously-normalizedphotomultiplier disengaged, and again the gain of the engagedphotomultiplier is adjusted if necessary to restore the measuredchannels ratio to within a desired tolerance of 1.000. If the ratiosobtained from each photomultiplier 24, 24" is within the acceptabletolerances after normalization, it can be concluded that the gain ofeach photomultiplier tube will be at its normal, or design, level andthat both of the photomultiplier tubes have substantially equal gains,i.e., are in balance. Under these conditions, the net external standardratio from external standardization unit 56 will be a function only ofthe degree of quench within the sample and scintillator (FIG. 4) and theposition, and hence counting efficiency, of a spectral curve will notexhibit a change caused by system gain (FIG. 6).

Exemplary Normalization Circuit An exemplary photomultiplier tubecircuit according to the invention, having an adjustable current gainwithout necessarily adjusting the normally-constant high voltage input,is depicted schematically in FIG. 9, certain portions of which werediscussed previously. Although, as earlier indicated, the high voltageinputactually inputs 42 and 45-may and conveniently are 1 madeadjustable in steps, the principal feature of the circuit of FIG. 9 isthat the gain of the photomultiplier tube 24' can be adjustedindependent of the high voltage power supply and independent ofadjustments of the other photomultiplier.

Turning once again to FIG. 9, the photomultiplier tube 24' comprises anenvelope containing a cathode 41, and anode 48, and a sequential arrayof dynodes D1 through D13, inclusive. The dynodes are connected througha voltage divider network including resistances Rl through R12 andresistances R14 and R13 so that each dynode is supplied with aprogressively higher potential relative to the cathode, which in thepresent embodiment is shown grounded. Further, a first high voltage dccurrent is supplied from source 42 to dynode D13 via resistance R14, asecond high voltage dc current is supplied to dynode D10, while a lowvoltage, volts dc, is connected to dynode D1; maintaining thisrelatively high potential between the cathode and the first dynodeindependently of the high voltage output settings minimizes the effectof noise due to random fluctuations that occur in the electronmultiplication process.

The only variable interdynode voltages are the potentials developedbetween dynodes D1 through D10, the potentials between dynodes D10 andD13 being maintained constant by maintaining a constant 1,000 voltdifference between the first high voltage source 42 and the second highvoltage source 45. Between each of dynodes D1 through D10 is aresistance sub-network where, preferably, an equal dc acceleratingvoltage may be applied to each interdynode stage.

An additional voltage divider network is provided between dynodes D2 andD4 composed of a pair of fixed resistances in series R10, R11, and aparallel circuit composed of the series resistances R18, R19, R20.Resistance R19 is an adjustable potentiometer. Thus, by adjustment ofthe slider 73 on potentiometer R19, the voltage between dynode D3 andits preceding dynode D2 (or, by difference, between D3 and itssucceeding dynode D4) can be adjusted without affecting the voltagebetween the preceding dynode D2 and the succeeding dynode D4. By thesame token, changing the voltage at dynode D3 does not affect thevoltages at the other dynodes and changes the interdynode gain onlybetween D2-D3 and between D3-D4. In other words, the gain at all otherstages of the photomultiplier tube 24, and perforce in all stages of thecorresponding photomultiplier tube 24" (FIG. 8) will be unaffected byadjustments of the voltage applied to dynode D3.

Inasmuch as a change in the potential between dynode D3 and D2concurrently changes the potential between dynode D3 and D4 (by anamount equal to the difference between the potential across D4 and D2less the potential between D3 and D2), a change in the potential appliedto dynode D3 affects the gain in two interdynode stages, namely D2-D3and D3-D4. The magnitude of each stage gain depends on the potentialacross. D4 and D2 as well as the adjustment made in the potentialbetween D3 and D2, and for a D4-D2 of 140 volts dc, the Table belowillustrates the magnitude of dynode 4-2 stage gains that can beobtained. Thus, a comparatively broad gain adjustment in thephotomultiplier tube 24 can be accomplished by adjusting the voltageapplied to one of the dynodes.

TABLE II DYNODE STAGE GAINS AS FUNCTION OF DYNODE VOLTAGE, FOR +140 V dcBETWEEN To disengage one of the photomultipliers while the other isbeing normalized as described previously, a system is provided thateffectively eliminates the gain of that photomultiplier which is notbeing normalized. This, moreover, is preferably accomplished withoutdisconnecting the high voltage inputs or the output of the disengagedphotomultiplier tube. To this end, the low voltage dc from source 46 ispassed through resistance R16 and is made available at terminal 72 ofthe switch 74. When the switch 74 is connected to terminal 72 a voltageis applied to the dynode D3 which is negative with respect to dynode D2.As a consequence, electrons normally attracted from D2 to D3 arerepelled by the now-negative potential and consequently aresubstantially prevented from further passage through the photomultipliertube 24. As a result, when switch 74 is connected to terminal 72 thephotomultiplier tube 24' is effectively producing no output pulses.

In similar manner, a corresponding circuit is provided for the opposedphotomultiplier tube 24" (FIG. 8). Thus, both tubes may be normalizedwithout physically disconnecting or disengaging either the high voltageinputs or the output circuitry.

One final point: a pair of terminals 75, 76 is provided for switch 74 sothat, when the switch is in engagement with either terminal 75 or 76,the position of wiper 73 on potentiometer R19 remains unaltered, andconsequently the potential applied to dynode D3 is unchanged. Switch 74may thus be ganged to a corresponding switch, not shown, in acorresponding circuit for the opposed photomultiplier 24" (FIG. 8), andwith the shunt 71 around attenuation unit 62 (FIG. 8). Thus, with switch74 in engagement with terminal 75,

both photomultipliers are operating and the system of FIG. 8 may be usedfor counting; with terminal 72 engaged by switch 74 the photomultiplier24' is disengaged while the photomultiplier 24" (FIG. 8) is beingnormalized. Similarly, with the switch 74 engaged with terminal 76, thephotomultiplier 24' may be normalized while photomultiplier 24" (FIG. 8)is disengaged.

Thus it is apparent that there has been provided, according to theinvention, a method and means that fully satisfy the objects, aims, andadvantages set forth earlier.

I claim as my invention:

l. A method of normalizing a spectrometer having (1) a pair ofphotomultiplier tube circuits responsive to scintillations fromradioactivity decay events for producing electrical signals proportionalin amplitude to the energy of corresponding decay events, each of saidcircuits comprising (a) a photomultiplier having an anode, a cathode,and a sequential array of dynodes, (b) a voltage divider networkinterconnecting said dynodes, and (c) means for applying a high voltageacross said network to establish progressively higher potentials at saiddynodes relative to the cathode, (2) means for amplifying the signalsfrom said circuit, (3) discriminator means for passing only a selectedamplitude band of amplified signals therethrough, and (4) means forcounting discriminated amplified signals as a measure of the activitylevel of a radionuclide, the improvement comprising:

1. disposing a radionuclide near each of said photomultipliers, saidradionuclide having predetermined count-producing characteristics,

counting the counts produced by said radionuclide, and

3. adjusting each of said voltage divider networks independent of oneanother and independent of said high voltage to thereby independentlyadjust the gain of each of said photomultiplier tube circuits to producea count substantially equal to said predetermined count so that saidspectrometer is thereby compensated for changes in gain characteristicsof said photomultipliers with time.

2. A method of normalizing a scintillation spectrometer having (1) apair of means'responsive to radioactivity decay events for producingelectrical signals proportional in amplitude to the energy ofcorresponding decay events (2) means for amplifying the signals fromsaid event-responsive means, (3) discriminator means for passing only aselected amplitude band of amplified signals therethrough, (4) 'circuitmeans adapted to selectively respond to simultaneous signals of bothsaid event-responsive means or to signals of one of saidevent-responsive means and for delivering an output signal, and (5)means for counting said output signals as a measure of the number ofsaid dec'ay events, which method comprises the steps of:

l. exposing a radionuclide having predetermined count-producingconditions to said event-responsive means, said radionuclide producingcounts at said counting means, and V adjusting the response of only oneof said eventresponsive means and then only the other of saidevent-responsive means to produce counts at said counting meanssubstantially equal to said predetermined count wherein said adjustmentof one of said event-responsive means is independent of the adjustmentof said other event-responsive means.

. A scintillation spectrometer comprising:

. a pair of means responsive to radioactivity decay events for producingelectrical signals proportional in amplitude to the energy ofcorresponding decay events, each of said pair of radioactivity decayevent-responsive means exhibiting changes in response characteristicswith time,

. means for applying a voltage to each of said pair of event-responsivemeans,

3. means for amplifying signals from each of said pair ofevent-responsive means,

discriminator means for passing only a selected amplitude band ofamplified signals therethrough, 5. means for counting discriminatedamplified signals as a measure of the number of said decay events,

. means for disposing near each of said pair of event-responsive means aradionuclide having predetermined count-producing characteristics, saidradionuclide producing counts at said counting means, and

7. means for adjusting each of said pair of radioactivity decayevent-responsive means independent of one another and independent of thevoltage applied to said event-responsive means to produce a count atsaid counting means substantially equal to said predetermined count.

4. Spectrometer of claim 3 including a plurality of discriminator means,one of said discriminator means being operative to produce counts whendetermining the activity level of an unknown sample and the other ofsaid means being operative to produce counts when said radionuclidehaving predetermined count-producing characteristics is disposed nearsaid event-responsive means.

, 5. Spectrometer of claim 3 wherein said radionuclide is agamma-emitting nuclide.

6. A scintillation spectrometer having provisions for adjusting the gaincharacteristics to compensate for gain changes with respect to timecomprising:

1. a pair of photomultiplier tube circuits responsive to scintillationsfrom radioactivity decay events for producing electrical signalsproportional in amplitude to the energy of corresponding decay events,each of said circuits comprising a. a photomultiplier having an anode, acathode,

and a sequential array of dynodes,

b. a voltage divider network interconnecting said dynodes, and

c. means for applying a high voltage across said network to establishprogressively higher potentials at said dynodes relative to the cathode,

2. means for amplifying the signals from each of said circuits,

3. discriminator means for passing only a selected amplitude band ofamplified signals therethrough,

. means for counting discriminated amplified signals as a measure of theactivity level of a radionuclide,

, 5. means for disposing a radionuclide having predeterminedcount-producing characteristics near said photomultipliers, saidradionuclide producing counts at said counting means, and

6. means for adjusting each of the voltage divider networks independentof one another and independent of the high voltage applied across saidnetworks to independently adjust the gain of said photomultiplier tubecircuits and thereby produce a count at said counting meanssubstantially equal to a predetermined normal count.

7. Spectrometer of claim 6 wherein said gain adjusting means includesmeans for adjusting the voltage applied to at least one of said dynodeswithout affecting the voltage between preceding and succeeding dynodes.

8. Spectrometer of claim 7 wherein said voltage dividing means comprisesa potentiometer connecting said preceding and succeeding dynodes, withthe potentiometer slider connected to said at least one of said dynodes.

9. Spectrometer of claim 7 including means for applying a biasingpotential to said at least one dynode lower than that of the precedingdynode, whereby the gain of said photomultiplier is reduced tosubstantially 0.

10. A scintillation spectrometer comprising:

1. a pair of photomultiplier tube means responsive to scintillationsfrom radioactivity decay events for producing electrical signalsproportional in amplitude to the energy of corresponding decay events,

. means for applying high voltage to each of said event-responsivemeans,

3. means for applying the signals from each of said event-responsivemeans,

. discriminator means for passing only a selected amplitude band ofamplified signals therethrough,

5. circuit means adapted to selectively respond to simultaneous signalsfrom both of said eventresponsive means or to signals from one of saidevent-responsive means and for delivering an output signal,

. means for counting output signals from said circuit means as a measureof the number of decay events, and

7. means operative when said circuit means responds to signals of onlyone of said event-responsive means for adjusting the response of saideventresponsive means to produce a count at said counting meanssubstantially equal to a predetermined count wherein said adjustingmeans adjusts the response of one of said event-responsive meansindependent of the response of the other eventresponsive means andindependent of said high voltage.

11. A scintillation spectrometer comprising:

1. a pair of photomultiplier tube circuits responsive to scintillationsfrom radioactivity decay events for producing electrical signalsproportional in amplitude to the energy of corresponding decay events,each circuit comprising:

a. a photomultiplier having an anode, a cathode,

and a sequential array of dynodes,

b. a voltage divider network interconnecting said dynodes, and

c. means for applying a high voltage across said network to establishprogressively higher potentials at said dynodes relative to the cathode,

2. means for amplifying the signals from said photomultiplier tubecircuits,

3. discriminator means for passing only a selected amplitude band ofamplified signals therethrough,

4. circuit means for selectively responding to simultaneous signals ofboth said photomultiplier tube circuits or to signals of one of saidcircuits and for delivering an output signal,

5. means for counting said output signals as a measure of the number ofsaid decay events, and

6. means operative when said circuit means responds to signals of one ofsaid photomultiplier tube circuits for adjusting said voltage dividernetwork independent of said high voltage to thereby adjust the gain ofsaid photomultiplier tube circuit to a predetermined normal gainindependent of the gain of the other photomultiplier tube circuit.

12. Spectrometer of claim 11 including amplifying means anddiscriminator means associated with each of said photomultiplier tubecircuits, and wherein said circuit means is subsequent to saidamplifying and discriminator means.

13. Spectrometer of claim 11 including means for summing the signalsfrom said photomultiplier tube circuits and for passing the summedsignals tosaid amplifying means, and wherein said circuit means includesa simultaneous signal responsive means prior to said amplifying anddiscriminator means. i

14. Spectrometer of claim 11 wherein said circuit means, when responsiveto signals of one of said photomultiplier tube circuits, includes twodiscriminator means to pass selected amplitude bands, and wherein saidcounting means includes means for determining the ratio of signalspassed through said bands as an indication of the gain of saidphotomultiplier tube circuits.

15. Spectrometer of claim 14 wherein said amplitude bands are integralchannels.

16. Spectrometer of claim 11 wherein said gain adjusting means isindependent of the high voltage applied to said photomultiplier.

l7. Spectrometer of claim 11 including means for applying a biasingpotential to one of said dynodes lower than that of the precedingdynode, whereby the gain of said photomultiplier tube circuit is reducedto substantially 0.

18. A coincident-counting scintillation spectrometer for detecting andmeasuring the activity of a radionuclide present in a sample,comprising:

1. first and second photomultiplier tube means responsive toscintillations from radioactivity decay events for producing electricalsignals proportional in amplitude to the energy of the correspondingdecay events,

2. first and second means for separately amplifying the electricalsignals fromsaid first and second photomultiplier tube means,

3. discriminator means connected to at least one of said amplifyingmeans for passing only a selected amplitude band of amplified signalstherethrough,

logic means responsive to discriminated simultaneous signals from saidfirst and said second amplifying means, 5. means for counting saiddiscriminated simultaneous signals as a measure of the activity of aradionuclide,

6. means for exposing a radionuclide having predeterminedcount-producing characteristics to said photomultiplier tube means, and

7. means for adjusting the gain of each of said photomultiplier tubemeans independent of the other of said photomultiplier tube means toproduce a count at said counting means substantially equal to saidpredetermined count.

19. A summation type, coincident counting, scintillation spectrometerfor detecting and measuring the activity ofa radionuclide in a sample,comprising:

l. first and second photomultiplier tube means responsive toscintillations from radioactivity decay events for producing electricalsignals proportional in amplitude to the energy of the correspondingdecay events,

. first logic means responsive to simultaneous signals from said firstand second photomultiplier tube means and for producing a coincidencesignal,

3. means for summing the electrical signals from said first and saidsecond photomultiplier tube means and for producing a first summationsignal,

. means for amplifying said first summation signal,

5. discriminator means for passing only a selected band of amplifiedsummation signals therethrough,

6. second logic means responsive to simultaneous signals from said firstlogic means and said discriminator means,

7. means for counting simultaneous signals from said second logic meansas a measure of the activity of a radionuclide,

8. means for exposing a radionuclide 5 having predeterminedcount-producing characteristics to said photomultiplier tube means, and

9. means for adjusting the gain of each of said photomultiplier tubemeans independent of the other of said photomultiplier tube means toproduce a count at said counting means substantially equal to saidpredetermined count.

20. A scintillation spectrometer having external standardization,comprising:

1. first and second photomultiplier tube means responsive toscintillations from radioactivity decay events for producing electricalsignals pro portional in amplitude to the corresponding decay events,

2. means for amplifying the electrical signals from said first andsecond photomultiplier tube means,

3. discriminator means for passing only a selected amplitude band ofamplified signals therethrough,

. means for counting selected amplified signals as a measure ofradioactivity,

5. means for disposing near said first and second photomultiplier tubemeans (i) a series of samples of known activity and varying degrees ofquench, (ii) a quenched unknown sample, and (iii) a quenched unknownsample and a gamma-emitting standard, to correct for counting efficiencyin said quenched unknown sample, and

6. means for adjusting the gain of one of said photomultiplier tubemeans independent of the gain of the other photomultiplier tube means.

21. A scintillation spectrometer having external standardization,comprising:

1. first and second photomultiplier tube means responsive toscintillations from radioactivity decay events for producing electricalsignals proportional in amplitude to the energy of the correspondingdecay events,

first logic means responsive to simultaneous signals from said first andsecond photomultiplier tube means and for producing a coincidencesignal,

3. means for summing the electrical signals from said first and secondphotomultiplier tube means and for producing a first summation signal,

. at least one amplification-discrimination means for amplifying saidsummation signal and for passing only a selected band of amplifiedsummation signals therethrough,

5. second logic means responsive to simultaneous signals from said firstlogic means and said amplification-discrimination means,

6. means for counting said second simultaneous signals,

. an additional amplification-discrimination means for amplifying saidsummation signal and for passing only a selected band of amplifiedsummation signals therethrough,

8. means for disengaging one of said photomultiplier tubes and forcounting amplified signals from said additionalamp]ification-discrimination means and produced by the other tube as ameasure of the gain of said one photomultiplier tube, and means foradjusting the gain of one of said photomultiplier tube means independentof the gain of the other photomultiplier tube means.

22. Spectrometer of claim 21 wherein said additionalamplification-discrimination means includes two integral discriminationchannels, and said counting means includes means for determining theratio of signals from said integral discrimination channels.

1. A method of normalizing a spectrometer having (1) a pair ofphotomultiplier tube circuits responsive to scintillations fromradioactivity decay events for producing electrical signals proportionalin amplitude to the energy of corresponding decay events, each of saidcircuits comprising (a) a photomultiplier having an anode, a cathode,and a sequential array of dynodes, (b) a voltage divider networkinterconnecting said dynodes, and (c) means for applying a high voltageacross said network to establish progressively higher potentials at saiddynodes relative to the cathode, (2) means for amplifying the signalsfrom said circuit, (3) discriminator means for passing only a selectedamplitude band of amplified signals therethrough, and (4) means forcounting discriminated amplified signals as a measure of the activitylevel of a radionuclide, the improvement comprising:
 1. disposing aradionuclide near each of said photomultipliers, said radionuclidehaving predetermined count-producing characteristics,
 2. counting thecounts produced by said radionuclide, and
 3. adjusting each of saidvoltage divider networks independent of one another and independent ofsaid high voltage to thereby independently adjust the gain of each ofsaid photomultiplier tube circuits to produce a count substantiallyequal to said predetermined count so that said spectrometer is therebycompensated for changes in gain characteristics of said photomultiplierswith time.
 2. adjusting the response of only one of saidevent-responsive means and then only the other of said event-responsivemeans to produce counts at said counting means substantially equal tosaid predetermined count wherein said adjustment of one of saidevent-responsive means is independent of the adjustment of said otherevent-responsive means.
 2. A method of normalizing a scintillationspectrometer having (1) a pair of means responsive to radioactivitydecay events for producing electrical signals proportional in amplitudeto the energy of corresponding decay events (2) means for amplifying thesignals from said event-responsive means, (3) discriminator means forpassing only a selected amplitude band of amplified signalstherethrough, (4) circuit means adapted to selectively respond tosimultaneous signals of both said event-responsive means or to signalsof one of said event-responsive means and for delivering an outputsignal, and (5) means for counting said output signals as a measure ofthe number of said decay events, which method comprises the steps of: 2.means for applying a voltage to each of said pair of event-responsivemeans,
 2. means for amplifying the signals from said photomultipliertube circuits,
 2. counting the counts produced by said radionuclide, and2. means for applying high voltage to each of said event-responsivemeans,
 2. means for amplifying the signals from each of said circuits,2. first and second means for separately amplifying the electricalsignals from said first and second photomultiplier tube means,
 2. firstlogic means responsive to simultaneous signals from said first andsecond photomultiplier tube means and for producing a coincidencesignal,
 2. means for amplifying the electrical signals from said firstand second photomultiplier tube means,
 2. first logic means responsiveto simultaneous signals from said first and second photomultiplier tubemeans and for producing a coincidence signal,
 3. means for summing theelectrical signals from said first and second photomultiplier tube meansand for producing a first summation signal,
 3. discriminator means forpassing only a selected amplitude band of amplified signalstherethrough,
 3. means for summing the electrical signals from saidfirst and said second photomultiplier tube means and for producing afirst summation signal,
 3. discriminator means connected to at least oneof said amplifying means for passing only a selected amplitude band ofamplified signals therethrough,
 3. means for applying the signals fromeach of said event-responsive means,
 3. discriminator means for passingonly a selected amplitude band of amplified signals therethrough,
 3. Ascintillation spectrometer comprising:
 3. means for amplifying signalsfrom each of said pair of event-responsive means,
 3. adjusting each ofsaid voltage divider networks independent of one another and independentof said high voltage to thereby independently adjust the gain of each ofsaid photomultiplier tube circuits to produce a count substantiallyequal to said predetermined count so that said spectrometer is therebycompensated for changes in gain characteristics of said photomultiplierswith time.
 3. discriminator means for passing only a selected amplitudeband of amplified signals therethrough,
 4. means for countingdiscriminated amplified signals as a measure of the activity level of aradionuclide,
 4. Spectrometer of claim 3 including a plurality ofdiscriminator means, one of said discriminator means being operative toproduce counts when determining the activity level of an unknown sampleand the other of said means being operative to produce counts when saidradionuclide having predetermined count-producing characteristics isdisposed near said event-responsive means.
 4. discriminator means forpassing only a selected amplitude band of amplified signalstherethrough,
 4. circuit means for selectively responding tosimultaneous signals of both said photomultiplier tube circuits or tosignals of one of said circuits and for delivering an output signal, 4.discriminator means for passing only a selected amplitude band ofamplified signals therethrough,
 4. logic means responsive todiscriminated simultaneous signals from said first and said secondamplifying means,
 4. means for amplifying said first summation signal,4. means for counting selected amplified signals as a measure ofradioactivity,
 4. at least one amplification-discrimination means foramplifying said summation signal and for passing only a selected band ofamplified summation signals therethrough,
 5. second logic meansresponsive to simultaneous signals from said first logic means and saidamplification-discrimination means,
 5. means for disposing near saidfirst and second photomultiplier tube means (i) a series of samples ofknown activity and varying degrees of quench, (ii) a quenched unknownsample, and (iii) a quenched unknown sample and a gamma-emittingstandard, to correct for counting efficiency in said quenched unknownsample, and
 5. discriminator means for passing only a selected band ofamplified summation signals therethrough,
 5. means for counting saiddiscriminated simultaneous signals as a measure of the activity of aradionuclide,
 5. circuit means adapted to selectively respond tosimultaneous signals from both of said event-responsive means or tosignals from one of said event-responsive means and for delivering anoutput signal,
 5. means for counting said output signals as a measure ofthe number of said decay events, and
 5. means for counting discriminatedamplified signals as a measure of the number of said decay events, 5.means for disposing a radionuclide having predetermined count-producingcharacteristics near said photomultipliers, said radionuclide producingcounts at said counting means, and
 5. Spectrometer of claim 3 whereinsaid radionuclide is a gamma-emitting nuclide.
 6. means for disposingnear each of said pair of event-responsive means a radionuclide havingpredetermined count-producing characteristics, said radionuclideproducing counts at said counting means, and
 6. means operative whensaid circuit means responds to signals of one of said photomultipliertube circuits for adjusting said voltage divider network independent ofsaid high voltage to thereby adjust the gain of said photomultipliertube circuit to a predetermined normal gain independent of the gain ofthe other photomultiplier tube circuit.
 6. means for counting outputsignals from said circuit means as a measure of the number of decayevents, and
 6. means for adjusting each of the voltage divider networksindependent of one another and independent of the high voltage appliedacross said networks to independently adjust the gain of saidphotomultiplier tube circuits and thereby produce a count at saidcounting means substantially equal to a predetermined normal count. 6.means for exposing a radionuclide having predetermined count-producingcharacteristics to said photomultiplier tube means, and
 6. Ascintillation spectrometer having provisions for adjusting the gaincharacteristics to compensate for gain changes with respect to timecomprising:
 6. second logic means responsive to simultaneous signalsfrom said first logic means and said discriminator means,
 6. means foradjusting the gain of one of said photomultiplier tube means independentof the gain of the other photomultiplier tube means.
 6. means forcounting said second simultaneous signals,
 7. an additionalamplification-discrimination means for amplifying said summation signaland for passing only a selected band of amplified summation signalstherethrough,
 7. means for counting simultaneous signals from saidsecond logic means as a measure of the activity of a radionuclide, 7.means for adjusting the gain of each of said photomultiplier tube meansindependent of the other of said photomultiplier tube means to produce acount at said counting means substantially equal to said predeterminedcount.
 7. Spectrometer of claim 6 wherein said gain adjusting meansincludes means for adjusting the voltage applied to at least one of saiddynodes without affecting the voltage between preceding and succeedingdynodes.
 7. means for adjusting each of said pair of radioactivity decayevent-responsive means independent of one another and independent of thevoltage applied to said event-responsive means to produce a count atsaid counting means substantially equal to said predetermined count. 7.means operative when said circuit means responds to signals of only oneof said event-responsive means for adjusting the response of saidevent-responsive means to produce a count at said counting meanssubstantially equal to a predetermined count wherein said adjustingmeans adjusts the response of one of said event-responsive meansindependent of the response of the other event-responsive means andindependent of said high voltage.
 8. means for disengaging one of saidphotomultiplier tubes and for counting amplified signals from saidadditional amplification-discrimination means and produced by the othertube as a measure of the gain of said one photomultiplier tube, andmeans for adjusting the gain of one of said photomultiplier tube meansindependent of the gain of the other photomultiplier tube means. 8.Spectrometer of claim 7 wherein said voltage dividing means comprises apotentiometer connecting said preceding and succeeding dynodes, with thepotentiometer slider connected to said at least one of said dynodes. 8.means for exposing a radionuclide having predetermined count-producingcharacteristics to said photomultiplier tube means, and
 9. means foradjusting the gain of each of said photomultiplier tube meansindependent of the other of said photomultiplier tube means to produce acount at said counting means substantially equal to said predeterminedcount.
 9. Spectrometer of claim 7 including means for applying a biasingpotential to said at least one dynode lower than that of the precedingdynode, whereby the gain of said photomultiplier is reduced tosubstantially
 0. 10. A scintillation spectrometer comprising:
 11. Ascintillation spectrometer comprising:
 12. Spectrometer of claim 11including amplifying means and discriminator means associated with eachof said photomultiplier tube circuits, and wherein said circuit means issubsequent to said amplifying and discriminator means.
 13. Spectrometerof claim 11 including means for summing the signals from saidphotomultiplier tube circuits and for passing the summed signals to saidamplifying means, and wherein said circuit means includes a simultaneoussignal responsive means prior to said amplifying and discriminatormeans.
 14. Spectrometer of claim 11 wherein said circuit means, whenresponsive to signals of one of said photomultiplier tube circuits,includes two discriminator means to pass selected amplitude bands, andwherein said counting means includes means for determining the ratio ofsignals passed through said bands as an indication of the gain of saidphotomultiplier tube circuits.
 15. Spectrometer of claim 14 wherein saidamplitude bands are integral channels.
 16. Spectrometer of claim 11wherein said gain adjusting means is independent of the high voltageapplied to said photomultiplier.
 17. Spectrometer of claim 11 includingmeans for applying a biasing potential to one of said dynodes lower thanthat of the preceding dynode, whereby the gain of said photomultipliertube circuit is reduced to substantially
 0. 18. A coincident-countingscintillation spectrometer for detecting and measuring the activity of aradionuclide present in a sample, compRising:
 19. A summation type,coincident counting, scintillation spectrometer for detecting andmeasuring the activity of a radionuclide in a sample, comprising:
 20. Ascintillation spectrometer having external standardization, comprising:21. A scintillation spectrometer having external standardization,comprising:
 22. Spectrometer of claim 21 wherein said additionalamplification-discrimination means includes two integral discriminationchannels, and said counting means includes means for determining theratio of signals from said integral discrimination channels.